With 24 inch range and needing to roll to hit this gun is pretty much only useful against one kind of target : small, heavily armored units. Mostly knights. You get one chance to shoot them, at long range, before they charge you. For everything else, mortars and canons are better.
At BS 2 on average you need the following to kill a single 2+ save t3 knight(heavy armor, shield, barded mount) : 2/3 chance to fail a save., 5/6 chance to wound, 1/3 chance to hit, or about 6 shots. This assumes no cover at all, or magic.
The maths is kind of complicated but...the maximum damage you can do is 30 shots, at bs 2, killing 5.56 t3 2+ save knights, but thats only a 0.5% chance of happening. Meanwhile theres a 1/6 chance of misfiring on the first shot.
It doenst look too good really. If its range was boosted to 36 inches and it could shoot at short range without getting charged, then maybe...
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